1. Field of the Invention
The present invention relates to generation of acoustic waves, and to imaging of subterranean formations. In another aspect, the present invention relates to methods and apparatus for generation of acoustic waves, and to methods and apparatus for imaging of subterranean hydrocarbon formations. In even another aspect, the present invention relates to methods and apparatus for producing shear ("S") waves, and to apparatus and methods for imaging the subterranean utilizing such S waves. In still another aspect, the present invention relates to producing S waves with an explosive packaging, and to methods for using such S wave sources in oil and gas exploration. In yet another aspect, the present invention relates to S waves produced with an explosive packaging that can be detonated in shot holes in a manner that produces directionally controlled force vectors, and to methods for using such S wave sources in oil and gas exploration.
2. Description of the Related Art
Seismic exploration generally involves generating seismic pulses at the surface of the earth by means of one or more seismic sources. The seismic pulses travel downwardly into the earth with a fractional amount being reflected and/or refracted due to differences in elastic properties at the interface of various subterranean formations.
Detectors, such as seismometers, geophones or hydrophones, produce analog electrical seismic signals or seismic trace signals in response to detected seismic wave reflections and/or refractions. The analog electric seismic signals or seismic trace signals from the detectors can then be recorded. Alternatively, the analog seismic signals or seismic trace signals from the detectors can be sampled and digitized prior to being recorded. The seismic data recorded in either manner are subsequently processed and analyzed to determine the nature and structure of the subterranean formations.
From the recorded data, a seismic section is generated. A seismic section is a seismic image depicting the subsurface layering of a section of earth along a seismic line of profile. The seismic section is an important tool which the geologist studies to determine the nature of the earth's subsurface formations. However, before an array of seismic samples can be converted into a seismic section which can be interpreted by the geologist, the seismograms must be processed to reduce the degradation due to noise.
Seismic interpretation generally involves the study of the behavior of arrival times, amplitudes, velocities, frequencies, and character of the reflections from target horizons. Any changing or anomalous behavior is of particular interest.
It is well known that there are benefits to imaging oil and gas reservoirs utilizing both compression ("P") waves and shear ("S") waves. To date, the bulk of S-wave data acquisition has been done using S-wave vibrators that shake their pads in a horizontal plane.
However, limitations in S-wave sources have not allowed 3-D S-wave data acquisition to be done in a practical, cost-effective manner.
Current S-wave sources are sometimes as much a barrier to S-wave imaging as they are an asset because they: (1) create excessive surface damage; (2) produce inconsistent source wavelets from source station to source station; (3) have narrow signal bandwidths; and (4) the current S-vibrators cannot be phase-locked to create effective source arrays.
Existing S-wave seismic sources cause so much surface damage that property over many attractive oil and gas prospects cannot be permitted for S-wave data acquisition. For example, it is well documented that each cleat of a typical S-wave vibrator pad, which usually has four to six cleats per pad, will cause surface damage on the order of 15 inches deep, 18 inches wide and 24 inches long. During seismic operations, it is not unusual to record and sum twenty or more sweeps at each source station, with the vibrator pad relocated for each of these sweeps, thus causing the ground damage to be repeated again and again across each source station location. After data are generated at a large number of source stations, the ground surface over the prospect takes on the appearance of a huge waffle cake. Landowners often refuse to allow such damage to their property, or they charge excessively high permitting fees for seismic access.
Impulsive S-wave sources, commercially available under the tradenames Omnipulse and ARIS, also create similar surface damage. In some instances, gravel pads are constructed at each source point so that the repeated pounding of the inclined weight used by these impulsive sources does not create a deep depression. While these gravel pads usually do reduce surface damage, they cause data acquisition expenses to increase significantly due to the cost and effort required to construct the gravel pads, and some landowners object just as much to gravel piles being on their property as they do to repeated surface depression damage.
A second deficiency of current S-wave surface seismic sources is that it is difficult (probably impossible in fact) to verify that there is a consistent pad-to-earth coupling at each source station. Consequently, there is no way to know that consistent source wavelets are produced at each source point, and consistent source wavelets are particularly critical if S-waves are to be used for accurate imaging of complex stratigraphy.
A third deficiency of current S-wave surface seismic sources is that the bandwidth of the generated waves is a relatively narrow 10 to 30 Hz. In contrast, properly designed explosive charges and shot holes can sometimes generate P-wave frequencies as high as 150 Hz.
Thus there is a need in the art for methods and apparatus for producing S waves.
There is another need in the art for methods and apparatus for producing S wave for imaging oil and gas reservoirs sources which do not suffer from the disadvantages of the prior art.
There is even another need in the art for methods and apparatus for producing S waves for imaging oil and gas reservoirs, which do not cause excessive surface ground damage.
There is still another need in the art for methods and apparatus for producing S wave sources for imaging oil and gas reservoirs that produce consistent wavelets from source station to source station.
There is yet another need in the art for methods and apparatus for producing S waves for imaging oil and gas reservoirs that produce a broader S-wave bandwidth.
These and other needs in the art will become apparent to one of skill in the art upon review of this specification.